A button-type alkaline battery has a configuration in which an anode, a separator, and a cathode are disposed in a sealed space formed by sealing a cathode housing with an anode sealing member, with a gasket being interposed between the cathode housing and the anode sealing member. In conventional button-type alkaline batteries, hydrogen gas may be generated during storage, and the generation of hydrogen gas then may cause the batteries to swell or deteriorate in capacity in some cases.
The generation of hydrogen gas is caused by the following two types of reactions. The first reaction is a self-corrosion reaction of zinc that is contained in the anode as an anode active material. Zinc dissolves in an alkaline electrolyte while generating hydrogen gas. The second reaction is a local cell reaction that is caused at the interface between the anode sealing member and the anode containing zinc as an anode active material. When zinc and copper that is contained in the anode-side surface (the inner surface) of the anode sealing member come into contact with each other in the presence of an alkaline electrolyte, a local cell is formed. This local cell reaction decomposes water contained in the alkaline electrolyte and thereby generates hydrogen gas. The generation of hydrogen gas that is caused by the local cell reaction continues until zinc that has dissolved in the alkaline electrolyte deposits on the inner surface of the anode sealing member to form a zinc layer on the inner surface of the anode sealing member. Zinc has a higher hydrogen overpotential than that of copper. Hence, when the zinc layer is formed, the generation of hydrogen gas that is caused by the local cell reaction is inhibited. However, the generation of hydrogen gas that is caused by the self-corrosion reaction of zinc contained in the zinc layer continues.
The conventional button-type alkaline batteries employ amalgamated zinc powder as an anode active material for the purpose of inhibiting the above-mentioned generation of hydrogen gas. When the amalgamated zinc powder is used as an anode active material, mercury whose hydrogen overpotential is higher than that of zinc inhibits the self-corrosion reaction of the zinc. In addition, when the amalgamated zinc powder is used as an anode active material, a zinc-mercury alloy layer is formed on the anode-side surface of the anode sealing member. The zinc-mercury alloy layer is formed through the solid phase diffusion of mercury in a zinc layer formed on the inner surface of the anode sealing member.
When the zinc-mercury alloy layer is formed on the inner surface of the anode sealing member, the generation of hydrogen gas that is caused by the local cell reaction is inhibited. Furthermore, the self-corrosion reaction of the zinc contained in the zinc-mercury alloy layer also is inhibited by the mercury contained in the zinc-mercury alloy layer. Accordingly, the generation of hydrogen gas that is caused by the self-corrosion reaction also is inhibited.
Recently, from the viewpoint of environmental pollution control, it has been demanded strongly to allow the anode active material to be free from mercury. A mercury-free zinc alloy powder that corrodes less has already been used practically as an anode active material. This zinc alloy powder contains zinc, as well as, for instance, aluminum, bismuth, or indium. The aluminum, bismuth, or indium improves the corrosion resistance of the zinc. Cylindrical alkaline batteries have a configuration in which an anode is not in contact with an anode sealing member. Hence, the problem of hydrogen gas generation that is caused by the local cell reaction does not arise. The above-mentioned zinc alloy powder therefore has been used already as an anode active material.
On the other hand, in the case of button-type alkaline batteries, when the above-mentioned zinc alloy powder simply is used instead of the amalgamated zinc powder, a problem arises that the batteries may swell and deteriorate in capacity particularly during high temperature storage.
When the above-mentioned zinc alloy powder is used as an anode active material, the self-corrosion reaction of the zinc contained in the zinc alloy powder included in the anode can be inhibited. Accordingly, the generation of hydrogen gas that is caused by the self-corrosion reaction of the zinc can be inhibited. The zinc contained in the zinc layer formed on the inner surface of the anode sealing member, however, tends to be re-corroded (re-dissolved) by an alkaline electrolyte. Hence, even when the generation of hydrogen gas that is caused by the local cell reaction can be inhibited through the formation of the zinc layer, hydrogen gas is generated through re-corrosion of the zinc contained in the zinc layer. This generation of hydrogen gas is accompanied by the problem that the batteries may swell and deteriorate in capacity. This problem becomes conspicuous particularly during high temperature storage of the batteries.
In order to solve the above-mentioned problems, it has been proposed to preform a metal layer containing metal whose hydrogen overpotential is higher than that of copper, such as, for instance, zinc, tin, lead, or indium, on the anode-side surface of the anode sealing member before the assembly of a battery. The above-mentioned metal layer can be formed by, for instance, plating or vapor deposition (see, for example, Patent Documents 1 to 4).    Patent Document 1: JP5(1993)-266881A    Patent Document 2: JP6(1994)-89724A    Patent Document 3: JP6(1994)-163026A    Patent Document 4: JP2000-156207A
However, in order to preform the metal layer on the inner surface of the anode sealing member before the assembly of a battery, special processes such as, for instance, a plating process are required.
The present invention provides a button-type alkaline battery in which mercury-free zinc or a mercury-free zinc alloy is used as an anode active material. In the button-type alkaline battery of the present invention, the generation of hydrogen gas is inhibited and the battery is prevented from swelling and deteriorating in capacity without using processes, such as a plating process, which are not employed conventionally.